Seems to me the mistake was believing Voyager was doing fine and only needed to replace one boring character with sexy Seven of Nine. In reality, half the cast had no personality and just stood around reading computer displays while the EMH character carried the show.
They should have taken the opportunity to do a more major overhaul of the show, getting rid of Kes, Kim, Chakotay, and perhaps Tuvok; and replaced them with characters with some interesting dramatic tension.